It is known that polymers are subject to strong electrostatic charging, and that, owing to the low electrical conductivity of polymers, charges once applied can be dissipated only at a slow rate. In addition to the aesthetic factors, however, safety factors often require rapid charge decay. Adverse effects during use which may be mentioned include: soiling of polymer surfaces, electrical shocks to persons coming into contact with polymers, disrupted production as a result of the sticking together of film webs, destruction of electronic components, clumping in polymer powders, and spark discharge as a result of excessive charge buildup with subsequent ignition, which has in the past occasionally led to severe explosions.
It is known that static charging can be restricted by the use of additives which enhance the surface conductivity. These substances, however, have the disadvantage that they are almost totally ineffective at low atmospheric humidity. It is therefore better to use additives which increase the volume conductivity. However, the substances known to increase the volume conductivity, examples being carbon black and metal powders, lower the mechanical properties of the polymers and cannot be used for transparent polymers. A further requirement, expressed with increasing frequency, is that additives should be environmentally unobjectionable.
Further details regarding antistatic additives and the mechanism of static charging can be found, for example, in the "Plastics Additives Handbook", edited by R. Gachter and H. Muller, Hanser Verlag, 3rd edition, 1990, pages 749-775.
In order to achieve permanent antistatic treatment, the proposal has already been made in DE 4 324 062 to coat materials having a large surface area, such as fibres, with a colourless semiconducting material, for example tin oxide. This coated material can then be mixed in with the polymer granules and processed together with them. However, the preparation of this coating is complex, since to prepare the semiconducting coating it is necessary to impregnate fibrous substrates with an aqueous salt solution, then to dry them, and to carry out subsequent thermal conditioning of the deposited salt. These chemical and thermal processes on the fibre may damage the fibre, with the consequence of a lower degree of conductivity than would be expected from the conductivity of the semiconductors. A further disadvantage is that the fibres bend under mechanical loading, and the brittle semiconductor coating can be damaged, thereby likewise impairing the conductivity.
An alternative option is described in DE 43 16 607, where it is proposed to add lubricating, adhesive or coating substances to metallized polymer fibres, as are already available commercially, and thereby to increase their electrical conductivity. However, metallized fibres are relatively expensive, are difficult to produce, and severely reduce the transparency of the polymer into which they are incorporated. Here too, mechanical damage to the conductive coating and even to the fibre (fracture) cannot be ruled out entirely.
Consequently, there continues to be a need for an additive system for increasing the volume conductivity, which has an antistatic action, is ecologically unobjectionable, is effective at low atmospheric humidity, is simple to prepare, which maintains over a long period the volume conductivity which it imparts to the polymer, and which can be employed without notable restriction in all commercially available polymers.